Page 180 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Systems of connection in 3-phase ac

Objectives: At the end of this lesson you shall be able to
• explain the star and delta systems of connection
• state phase relationship between line and phase voltages and current in a star connection delta connection
• state the relationship between phase and the voltage and current in star and delta connection.

Methods of 3-phase connection: If a three-phase load is connected to a three-phase network, there are two
basic possible configurations. One is `star connection’ (symbol Y) and the other is `delta connection’ (symbol D ).
Star connection: In Fig 1 the three-phase load is shown as three equal magnitude resistances. From each
phase, at any given time, there is a path to the terminal points U, V, W of the equipment, and then through the
individual elements of the load resistance. All the elements are connected to one point N: the `star point’. This
star point is connected to the neutral conductor N. The phase currents iU, iV, and iW flow through the individual
elements, and the same current flows through the supply lines, i.e. in a star connected system, the supply line
current (IL) = phase current (IP).
The potential difference for each phase, i.e. from a line to the star point, is called the phase voltage and designated
as VP. The potential difference across any two lines is called the line voltage VL. Therefore, the voltage across
each impedance of a star connection is the phase voltage VP. The line voltage VL appears across the load
terminals U-V, V-W and W-U and designated as VUV, VVW and VWU in the Fig 1. The line voltage in a star-
connected system will be equal to the phasor sum of the positive value of one phase voltage and the negative
value of the other phase voltage that exist across the two lines (Fig 2).

Thus
V L = V = (phasor V ) - (phasor V )
UV
VN
UN
= phasor V + V .
VN
UN
Fig 1 Fig 2

In the phasor diagram (Fig 3)
V = V = V Cos 30°+ V Cos 30°
L UV UN NV
3
But Cos 30° =
2
Thus as V = V = V P
UN
VN
V = V .
3
P
L
This same relationship is applied to V , V and V .
UV VW WU

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CITS : Power - Electrician & Wireman - Lesson 26-29